Modern high-performance video and television displays, especially plasma and liquid-crystal (LCD) displays, are adapted to receiving digital signals corresponding to the information to be displayed. These digital inputs indicate the intensity, typically by component, to be displayed by each picture element (“pixel”) of the display. For example, modern component video signals include a component value for each of the pixel attributes of luma (“Y”), chroma-blue (“Cb” or “U”), and chroma-red (“Cr” or “V”). As a result, modern high resolution displays are able to render high fidelity images at real time data rates, having over one thousand pixels in each dimension with each pixel responsive to as much as a twenty-four bit digital signal.
As known in the art, video inputs are communicated and stored in a wide variety of formats. Broadcast television signals are still communicated in the analog domain, and these analog signals are communicated according to multiple standards around the world. In addition, video signals from other sources are now also available for display on digital displays. These other sources include cable and satellite digital video transmissions, video cameras, and video playback devices such as DVD players and video cassette recorders. In any case, these signals are often in the form of “composite” video signals, in that the color signals are communicated in the form of “luma” and “chroma” (or color difference) signals, rather than as intensity levels for each of the primary component colors. These signals are also typically analog signals. Examples of the applicable standards for conventional video signals include the well-known NTSC (National Television Systems Committee), PAL, and SECAM composite video signal standards.
Video decoder functions are now commonly used in many high-performance digital display and television systems for receiving video signals from these various sources and converting the video signals into a digital form for display. For example, a so-called “set-top box” for receiving cable or satellite digital video transmissions and for driving a digital video display typically includes a video decoder function. Modern set-top boxes also often have auxiliary inputs for receiving video signals from other sources, from which the video decoder in the set-top box generates the digital video output signals. Other systems that include a video decoder function include video decoder cards for personal computers, personal video recorders (PVRs) for digitally recording broadcast, cable, or satellite transmissions for later viewing, digital video projectors, digital VCRs and DVD recorders, video or home theater receivers, and indeed digital television sets and computer displays that are themselves (i.e., without an external set-top box) capable of digitally displaying video output from conventional analog input signals.
In the decoding of input signals into digitally displayable output signals, conventional video decoders typically apply gain to the decoded signal, so that the digital output signals have amplitudes that fit well within the dynamic range of the display device. Conventional video decoders apply this gain by way of automatic gain control (“AGC”), which in its general sense amplifies a varying input voltage using a gain that depends on the input voltage itself. AGC circuits and functions thus automatically control the amplifier gain so that the output voltage remains constant or within a predetermined dynamic output range.
In conventional video decoders for converting analog video signals into digital display signals, the AGC function measures the amplitude of the analog signal at a known time within the periodic signal, and adjusts the gain of its output signal based on that amplitude. Typically, conventional video decoders sample the analog video signal at its synchronization (“sync”) level, which is a portion of the analog signal in the horizontal blanking interval that is not displayable by the display but which is used to synchronize the displayable portions of the signal with the display scan lines. The sync “height”, or amplitude (i.e., the difference between the sync level and a reference level, such as the “back porch” level), is used in these conventional video decoders as the control input to an AGC function, in response to which the AGC gain of the video decoder is set.
Generally, the sync amplitude may not be representative of the actual amplitude of the video information to be displayed. For example, in the well-known NTSC standard, the nominal sync amplitude is −40 IRE, but this sync pulse is frequently compressed or clipped. If this clipping occurs, the clipped or compressed sync pulse received by the digital video decoder will be at a lower amplitude than its ideal amplitude. The AGC function in the conventional video decoder will, as a result, set its gain undesirably high in an attempt to compensate for the lower amplitude sync height, but this gain will be too high for the video signal itself (which was not clipped). The resulting images displayed will tend to be saturated, or too bright.
FIG. 1a illustrates a graph of a modulated chroma signal versus time. The overall amplitude of a modulated chroma signal is amplified by a gain based upon a color burst component of the modulated chroma signal. However, due to nonstandard video, the color burst can be very small compared to the other portions of the chroma signal causing oversaturation. Thus, the amplified modulated chroma signal can be clipped between a first boundary 12 and a second boundary 14. Any amplitudes outside this range are clipped to the maximum boundaries 12 and 14. This in turn can cause vertical bands shown in a respective display at places where clipping occurs.
FIG. 1b illustrates a graph of the chroma-blue (“Cb”, “Pb”, or “U”) and chroma-red (“Cr”, “PR”, or “V”) values for video data, where clipping occurs due to the chroma gain of the video data. The displayable Cr and Cb levels are within a window 30. However, an amplified chroma value 34 may be outside this window 30. Thus, the amplified chroma signal will be clipped to the chroma value 32. By doing so, the original hue level 36 is transformed to a hue level 38 that correctly reflect the encoded video, thereby being decoded erroneously. Therefore, there remains a desire to provide new methods for chroma automatic gain control that can minimize clipping of the chroma signal.